AT&T unlimited-data-plan subscribers who exceed 3GB worth of data on their 3G smartphones -- usually due to streaming music and videos -- will see a reduction in their transfer speeds, the wireless carrier announced Thursday.

The reduction in speed, known as throttling, also affects users who run their devices on 4G LTE networks. Those subscribers who exceed 5GB of data will experience slower speeds.

T-Mobilealready slows down data transfers when users exceed 2GB. Verizon Wireless throttles speeds for the top 5 percent of data users when they are connected to a congested cell site.

Sprint-Nextel does not slow speeds for unlimited data subscribers.

An Associated Press report shows about 17 million of AT&T’s customers with unlimited data plans could experience slower speeds. AT&T previously throttled the top 5 percent of large-data users on their unlimited data plans using a sliding scale system that affected those who fell below the 3GB threshold.

Last week, a judge in Southern California awarded an iPhone user $850 after he filed a small claims case against AT&T in January. The ruling stated AT&T wasn't being fair when it purposely slowed down his iPhone's unlimited data plan when he used less than 3GB of data in a billing cycle.

The wireless carrier's support page shows the company will send a text message to those high-data users stating, "Your data usage is near 3GB this month. Exceeding 3GB during this and future billing cycles will result in reduced data speeds, though you will still be able to email & surf the Web. Wi-Fi helps you avoid reduced speeds."

The text message also directs AT&T users to the carrier's page for tiered data plans.

AT&T suggests users connect to a Wi-Fi network, which doesn't count toward the threshold, to reduce data activity on their plans.

Users can also monitor their data usage by dialing *data# on their cellphones.